Tasks such as mowing lawns are tedious and can be unpleasant, but are necessary. A common fantasy depicts a homeowner relaxing in a hammock with a cool drink on a warm summer day, possibly in the shade of a tree, while a robotic device mows the lawn.
The prior art includes a number of patents issued for robotic lawnmowers. U.S. Pat. No. 4,777,785, issued on Oct. 18, 1988 to Rafaels, describes a method of guiding a robotic lawnmower that relies on pairs of sensors, one of which emits and one of which detects electromagnetic radiation. U.S. Pat. No. 4,887,415, issued on Dec. 19, 1989 to Martin, describes a robotic lawnmower that relies on infrared obstacle detectors to provide guidance signals. U.S. Pat. No. 5,163,273, issued on Nov. 17, 1992 to Wojtkowski et al., describes a robotic lawnmower that relies on a buried wire to provide guidance. U.S. Pat. No. 5,974,347, issued on Oct. 26, 1999 to Nelson, describes a robotic lawnmower that relies on a plurality of radio transmitters to provide guidance signals. U.S. Pat. No. 6,009,358, issued on Dec. 28, 1999 to Angott et al., describes a robotic lawnmower that relies on a plurality of transceivers, one that transmits signals having different propagation velocities, and one that receives the signals. German Patent No. DE3918867, which was published on Oct. 19, 1989, also describes a robotic lawnmower that employs buried iron bars as a guidance system. Friendly Robotics is the assignee of U.S. Pat. Nos. 6,255,793, 6,339,735, 6,417,641, 6,443,509, and 6,493,613, and U.S. Design Pat. No. D451,931, directed to robotic lawnmowers that use proximity sensors to detect predefined boundaries.
The manual cutting of an edge is a variation on the installation of boundaries, paths, buried wires, or transmitters. Some robotic lawn mowers rely on distinguishing the cut height of grass from the uncut, taller grass, and following the edge. One example is described in U.S. Pat. No. 4,133,404, issued Jan. 9, 1979 to Griffin. A manually cut edge or border is simply another predefined boundary or path, one that needs to be “reinstalled” before each occasion when the grass is to be cut.
One problem that is common to each of the robotic systems described above is the need to provide and to locate transmitters or other indicators of a desired path or boundary. The necessity to place such transmitters or other locators involves considerable expenditure of time, effort, and funds, and may require precise measurements over considerable distances. Alteration of the desired actions of the robotic apparatus may require further time, effort, and funds to change the configuration of the previously defined path or boundary. There is a need for a robotic apparatus such as a lawnmower that can operate autonomously without the necessity to define either a path or a boundary by the placement of transmitters or other indicators.